The technical field of the invention is more particularly the field of off-shore oil production in regions presenting extreme ocean and weather conditions, and in particular in Arctic or Antarctic regions, and working from floating supports.
In general, an oil production floating support has anchor means to enable it to remain in position in spite of the effects of currents, winds, and swell. It also generally includes drilling means, oil storage means, and oil processing means, together with means for off-loading to off-loading tankers, which tankers call at regular intervals to take away the production. Such floating supports or ships are conventionally referred to as floating production storage off-loading (FPSO) vessels or indeed as floating drilling & production units (FDPU) when the floating support is also used for performing drilling operations with wells that are deflected in the depth of the water. The abbreviation FPSO is used below.
When weather and sea conditions, i.e. swell, wind, and current are severe or even extreme, as during storms, it is preferred to anchor the FPSO via a turret, generally situated in known manner in the front half of the ship and on its axis, with the ship being free to turn about said turret under the effect of the wind, current, and swell. Thus, with wind, current, and swell exerting specific forces on the hull and the superstructures, the FPSO makes use of its freedom to turn about the vertical axis ZZ so as to put itself naturally in a position of least resistance. The pipes connecting it with the well heads are generally connected to the underside of the turret and they are connected to the FPSO via a rotary joint lying on the axis of said turret. When weather conditions might become extreme, as in the North Sea, in the Gulf of Mexico, or in the Arctic or the Antarctic, the FPSO is generally disconnectable so as to be capable of taking shelter and waiting for acceptable operating conditions to return.
The present invention relates more particularly to a floating support for off-shore oil production in the Arctic or the Antarctic, the support being fitted under its hull with a disconnectable turret from where there extend anchor lines connected to the sea bottom and bottom-to-surface connection pipes, said hull including in its longitudinal direction substantially plane sides that extend vertically, and possibly also in conventional manner bow and stern portions (at the front and rear ends of the ship) that are inclined relative to the horizontal and that are preferably shaped so as to form a reinforced pointed stem capable of breaking pack ice merely by bending it whenever said pack ice forces it way under said reinforced stem.
Floating supports advantageously present a hull with substantially vertical longitudinal sides in order to optimize their oil storage capacities, and also to obtain better behavior in heavy sea. However a hull with vertical sides is particularly disadvantageous in terms of behavior relative to pack ice. Thus, in U.S. Pat. No. 4,102,288 and U.S. Pat. No. 4,571,125, floating supports are proposed that present, amongst other means, sides with profiles that are curved or inclined so as to enhance ice breaking in the manner that is known for a ship's bow having a stem that slopes relative to the horizontal.
In known manner, an oil production floating support including a releasable mooring system of anchor lines anchored to the sea bottom and of bottom-to-surface connection pipes comprises:                a mooring buoy for said lines and bottom-to-surface connection pipes, said buoy preferably being an annular buoy; and        said mooring buoy being fastened under the hull of the floating support to a rotary device having a tower-shaped structure referred to as a “turret”, said turret being fastened to the hull within a cavity passing through the entire height of the hull of the floating support, said turret being rotatably mounted relative to said hull via at least one rolling or friction bearing, preferably a rolling bearing, so as to allow said floating support to turn about a substantially vertical axis ZZ′ of said turret and of said cavity without causing said mooring buoy to turn relative to the same vertical axis ZZ′; and        said bottom-to-surface connection pipes rise within the cavity to a coupling for a plurality of said pipes, said coupling being secured to the floating support level with the deck of the floating support, said coupling being rotatably mounted so as to allow said floating support to turn without turning said coupling that is referred to as a rotary joint coupling.        
In the above-described prior art, the rolling bearing is located either level with the deck of the floating support, or else in the bottom portion under water, i.e. the bearing is immersed, or indeed a combination of the above two configurations may be used.
Embodiments in which the rolling bearing is located solely level with the deck are suitable only for floating supports of relatively small height, in particular less than 15 meters (m). With greater heights, for floating supports having a height lying in the range 20 m to 25 m, in particular, the horizontal force on the turret resulting from the floating support turning gives rise to the structure of the turret bending along its length, thereby mechanically stressing the top rolling bearing and thus mechanically endangering its reliability of operation. Furthermore, when the rolling bearing is underwater in the bottom portion of the turret, this immersion affects the operating reliability and the durability of said rolling bearing, and above all gives rise to difficulties in performing maintenance operations. On-site action requires the use of divers and of considerable technical means, and it is generally necessary to perform such operations in a protected zone, such as a fjord, or better still in a dry dock, after the FPSO has been disconnected. Thus, when an FPSO is intended to remain in position for several tens of years without any programmed maintenance disconnections in dry dock or in a protected site, that type of turret is not suitable.
Supports of the above-defined type are known from GB 2 291 389 and EP 0 259 072.
WO 94/15828 describes a system for quickly connecting and disconnecting a mooring buoy, in which the mooring buoy has a top portion that is connected to the bottom of the hull of the floating support, more precisely via a mooring cavity that extends annularly at the bottom end of a cavity passing through the entire height of the hull of the floating support with the bottom-to-surface connection pipes passing up therethrough. The mooring buoy also has a bottom portion to which there are moored the bottom portions of bottom-to-surface connection pipes extending to the sea bottom, said bottom portion of the mooring buoy being rotatably mounted by means of a rolling bearing that is completely immersed, enabling said bottom portion to turn relative to the top portion of the mooring buoy secured to the hull.
That type of system with a completely immersed rotary portion and rolling bearings that are completely immersed is not suitable for mooring a large number of bottom-to-surface connection pipes, for which it is desirable to propose a system in which at least some of the rolling bearings are situated out of the water so that they can be maintained more easily and so that they can be implemented in operating conditions that are less constraining.
In WO 94/15828, provision is made at the bottom of the hull for internal tanks presenting a large area in horizontal section in which atmospheric pressure or preferably a vacuum is established, said internal tanks presenting a large area of contact in horizontal section with the top portion of the mooring buoy, with the buoy being designed to be fastened thereagainst. For this purpose, an interstitial annular zone is created between the mooring buoy and the tank at atmospheric pressure at the bottom of the hull of the ship, which zone is defined by two concentric annular gaskets, which annular zone of small volume is put into contact with the chamber at atmospheric pressure at the bottom of the hull of the ship in order to create positive buoyancy for the assembly constituted by the mooring buoy and the anchor lines and the bottom-to-surface connection pipes that are pressed against said contact area.